The present invention relates to low gloss (matte) additives for use in UV curable overprint varnish coatings applicable to various printing ink applications such as flexographic, screen and offset. More particularly, the present invention relates to UV curable low gloss overprint coating compositions containing a low gloss additive comprising polyamide polymers and a liquid, uncured, Michael addition resin and the cured coatings therefrom. The coatings of the present invention are UV curable with reduced levels of photoinitiator when compared with traditional UV curable, acrylate, low gloss overprint systems.
UV curable acrylate coatings(overprint varnishes) are 100% solids. These types of coatings inherently form a coating that has a high gloss. It is difficult to obtain a low gloss cured coating from UV curable acrylate compositions. Traditional low gloss or matte UV curable acrylate overprint varnish formulations use inorganic materials such as silica dispersed in acrylate monomers to reduce gloss and obtain matte finishes. Disadvantages of these systems include the use of high levels of photoinitiator to obtain a cured system under standard curing conditions (300 mj/cm2). The use of high levels of photoinitiators yield a high gloss finished product. There are also viscosity problems caused in part from using silica as a low gloss additive. A silica containing overprint system's application viscosity is difficult to control. It is not uncommon for resultant problems to generate batch rejection rates of 30% to 40% and raw material overruns of 20% to 30%.
Attempts to address the problems related to high gloss and application viscosity have been made by formulating an overprint varnish containing polyamides as a low gloss additive dispersed in a mixture of acrylates. Although the viscosity problem for the most part was obviated by substituting polyamide powders for silica in UV curable acrylate systems the gloss problem associated with the presence of relatively large amounts of photoinitiators still remained.
According to the invention it was found that the gloss problem attributable to the presence of relatively large amounts of photoinitiator could be solved without affecting improvements in viscosity by using a low gloss additive combination comprising dispersions of polyamides in a UV curable acrylate overprint varnish formulation in conjunction with liquid, uncured Michael addition resins. The polyamide and liquid, uncured Michael addition resin are added to overprint varnish coating formulations where a low gloss finish is desired in the cured coating. The amount of external photoinitiator in an overprint formulation can be significantly reduced by using the Michael addition resin technology described in U.S. Pat. Nos. 5,945,489 and 6,025,410 both assigned to Ashland Inc. and the contents of which are incorporated by reference herein in their entirety. Because the above referenced Michael addition resins are UV curable at typical UV curing conditions (<300 mj/cm2) with little or no photoinitiator, the amount of photoinitiator overall in the overprint varnish formulations is significantly reduced when compared to previous UV curable acrylate, low gloss, overprint varnish formulations.